1. Field of the Invention
The present invention relates in general to a new and improved method and apparatus for use in drilling boreholes into subsurface formations. More particularly, the present invention relates to a new and improved method and apparatus for use in determining and monitoring the downhole depth from which borehole cuttings are removed and are being received at the surface of the drilling location
2. Description of the Prior Art
It is known that when drilling a well, particularly those related to the rotary drilling of an oil or a gas well, a fluid--often called "drilling mud"--is injected into the drill pipe assembly from the surface of the earth. The fluid, or mud, is pumped downwardly through the drill pipe assembly to the bottom of the borehole, where it passes through the orifices in the drill bit and then flows upwardly toward the surface of the earth through the annular space between the drilling pipe assembly and the wall of the borehole. Upon the return of the mud to the surface of the earth, the mud is passed through a shaker screen which momentarily retains the stony refuse material (borehole cuttings) for evaluation and the mud is gathered in a storage tank from which the mud is reintroduced to the drilling pipe assembly. The drilling mud is commonly circulated and partially retained in the borehole for various reasons, as for example, to exert hydrostatic pressure to keep the subsurface pressure substantially sealed in the borehole and to remove the borehole cuttings from the bottom of the borehole up to the surface of the earth.
When an oil well is being drilled, one of the first methods of evaluating the exploratory activity is to look at the borehole cuttings and examine them from a geological point of view, as for example, what age are the cuttings, what type of rock is in the cuttings, do the cuttings contain a hydrocarbon, etc. When examining and evaluating the borehole cuttings, it is important to know the depth from which the borehole cuttings were removed from the earth. It will be appreciated that the borehole cuttings at the bottom of the borehole do not instantly appear at the surface of the earth but appear at the surface at some later time depending upon such things as the depth of the drill bit at the time the borehole cuttings are brought into existence from the action of the drill bit against the earth, the rate of the flow of the drilling mud, etc.
In the prior art, one method used to determine the depth from which the borehole cuttings originated is to stop the descent of the drill bit, introduce a predetermined amount of carbide into the downhole flow of drilling mud, record the time of the introduction of the carbide, monitor the discharge of the drilling mud at the surface of the earth for the presence of acetylene gas and record the time at which the presence of the acetylene gas occurs. The total elapsed time provides the time required for the drilling mud to traverse the round trip to the bottom of the borehole and back to the surface of the earth. Since the inside volume of each section of drill pipe is known, the total inside volume of the drill pipe down to any depth is known or can be easily calculated. The output of the mud pump at certain predetermined speeds is known (or may be determined at any particular speed of operation) so the amount of time required for, the mud pump to pump the carbide down to the drill bit (and the borehole cuttings) may be calculated. The amount of time can also be considered as the time necessary for the mud pump to pump sufficient mud to fill the inside volume of the drill pipe down to the drill bit. The time required for the borehole cuttings at the drill bit to travel to the surface of the earth from the drill bit (or place of origination) is equal to the total elapsed time minus the time required for the carbide to travel down to the drill bit. If the round trip for the drilling mud took one hour and twenty minutes at a depth of five thousand feet and the calculated time for the carbide to reach the drill bit was twenty minutes, then the borehole cuttings from that general depth would take approximately one hour to travel from the bottom of the borehole to the surface of the earth where the borehole cuttings could be observed. The time required for the borehole cuttings at the bottom of the borehole to reach the surface of the earth is known as the lag time. Obviously, this method depends on constant and continuous pumping of the mud pump.
The present invention as claimed is intended to provide a method and apparatus which eliminates many of the prior art deficiencies which include the necessity to operate the mud pump or mud pumps at the same speed at all times which includes during the calibration or initialization of the determination of the lag time, during the time borehole cuttings are being brought to the surface of the earth to be evaluated, etc. If the mud pump or mud pumps are not operated at the same speed at the times associated with the determination of the lag time then error will be introduced into the determination of the lag time. In the prior art, it is necessary to monitor and maintain a record, mental or written, of the time or times associated with the determination of lag time. In the prior art, lag time is a function of time and requires the measurement of time.